OBJECTIVES: (1) Electrophoretic studies on serum proteins and immunoglobulins in mice infected with M. leprae and M. lepraemurium and on sera from leprosy patients in various stages of infection indicated that the levels of both in M. leprae infected mice were quite similar to those observed in patients with early type of leprosy, whereas those seen in M. leprae-murium infected mice were quite similar to those observed in patients with advanced type of leprosy(2). Data on immune response in mice infected with M. marinum have shown significant differences in their immune response when compared with that seen in M. leprae infected mice, thus indicating that the M. marinum mouse model may not be the most suitable one for extrapolation of immune response in M. leprae infection. (3)Studies on the Indian Cancer Research Center (ICRC) Bacillus gave no clues as to placement of this organism in the spectrum of mycobacterial species used in our laboratory for comparative analysis. (4)The newly employed method of isoelectric focusing to obtain antigens in separated forms has shown promise of being a more effective way of approaching such studies. This method has yielded a number of fractions. When tested by comparative immunodiffusion methods, some of these fractions have been shown to possess single antigens.